1. Field of the Invention
The present invention generally relates to the formation of holes in thin sheet material (e.g., Kapton web) by punching and, more particularly, to the monitoring and control of punching tools operated at high speed, especially as used in the fabrication of electronic circuit components.
2. Description of the Prior Art
The manufacture of many articles involves the formation of holes therein. In particular, it has been the common practice in the construction of electrical and electronic devices to mount components on a perforated insulating board or other substrate by passing leads through holes in the board. In such a case, connections are often formed by a conductive metal pattern on the board and also by wiring passed through the board through holes, or vias formed in the boards, as in constructions involving the well-known printed circuit.
The holes, or vias, are extremely small and very numerous. Therefore, the apertures which form the vias must be accurately positioned and must be of very accurate geometry. To achieve this accuracy, a punch is generally used to form the vias. A punch is also desirable because the material from the punched hole will be of a relatively large particle size in comparison with other methods which cut or erode material from the aperture.
A punch apparatus is very large in comparison to the holes formed in order to develop the force necessary to successfully and cleanly punch through the thin sheet material. In an automated punch tool operation, reliable and high rate punch actuation is desirable to permit high productivity and production rates.
In a typical punch, an electromagnetic repulsion actuator exerts a force on the punch propelling it through the thin sheet, thereby forming vias. A preloaded compression spring returns the punch to its original position. Because the majority of the energy of the actuator is stored in the spring during its actuation, only about 5% of the energy is used to punch the thin sheet material. This causes increased cycle time which can be reduced by applying more energy to the coil. Increased energy allows the punch to return more quickly to its original position.
However, if excess energy is applied to the coil, the spring will not stop the actuator motion, thereby causing the reciprocating portion of the punch to strike other portions of the punch structure at the ends of the punching stroke and elastically rebound therefrom. This rebounding causes wear debris to be formed due to metallic collision which could destroy the punch stem by bending and deforming or breaking it. The wear debris also causes frictional changes which can, in the most severe cases, bind the punch when the friction force exceeds the spring force and prevent the return of the punch. These frictional changes lead to changes in performance. The product of frictional drag force and the distance it acts over represents a loss of energy of the punch during actuation. The loss of energy increases cycle time of the punch and increases the time in which the punch stays in the material, thereby causing hole elongation when punching a fixed film with a moving die bar/punch actuator array. In most failures, variable degrees of hole elongation in the thin material precede punch sticking in the material.
These shortcomings and problems may be reduced or overcome with a net improvement in operating speed coupled with reduced occurrences of metallic collisions. U.S. Pat. No. 5,410,233 to Carbaugh et al. describes an electronic solution of energy absorption, whereby coil current is applied to decelerate the punch during its return in order to reduce metallic collision. However, this is an expensive and complex solution.